A trench MOSFET (Metal Oxide Semiconductor Field Effect Transistor) has been used as a non-isolated step-down switching power supply (hereinafter, called VR: Voltage Regulator) which supplies power to a CPU (Central Processor Unit) of a personal computer or a server (for example, see Japanese Patent Application Laid-Open Publication No. 2008-218711 (Patent Document 1) and Japanese Patent Application Laid-Open Publication No. 2005-57050 (Patent Document 2)).
The trench MOSFET has a smaller cell pitch than that of a planar MOSFET (for example, see J. Ng et al., “A Novel Planar Power MOSFET With Laterally Uniform Body and Ion-Implanted JFET Region”, IEEE Electron Device Letter, 2008, vol. 29, no. 4, pp. 375-377, April 2008 (Non-Patent Document 1)) and thus has a large channel width Wg per unit area and there is an advantage as it can reduce ON resistance. However, there is a demerit of large feedback capacitance as it has a large facing area of a trench gate and a drain region.
In recent years, for achieving a higher current and a lower voltage of a CPU, the number of output capacitors, which are provided for suppressing voltage fluctuations of a CPU occurring when current consumption of the CPU is abruptly changed, has been increased and it causes increases in the size and cost of a VR.
It has been known that an improvement in switching frequency of a VR is effective to reduce the number of output capacitors (for example, Y. Ren et al., “Analysis of the power delivery path from the 12-V VR to the microprocessor”, in Proc. IEEE APEC' 04, 2004, vol. 1, pp. 285-291 (Non-Patent Document 2) and M. Xu et al., “Small signal modeling of a high bandwidth voltage regulator using coupled inductor”, IEEE Trans. Power Electron., vol. 22, no. 2, pp. 399-406, March 2007 (Non-Patent Document 3)).
A bottleneck in improving switching frequency is that temperature of a MOSFET exceeds an upper limit (for example, 150° C.) of usage temperature of the MOSFET due to loss occurring along with switching. Examples of the loss occurring upon switching are, as to a high-side MOSFET of a VR, turn-on loss, turn-off loss, and drive loss; and, as to a low-side MOSFET, there are conduction loss and recovery loss of a build-in diode, and drive loss. Among these examples of loss, the turn-on loss and turn-off loss of a high-side MOSFET account for a relatively large portion. Hereinafter, the turn-on loss and turn-off loss will be collectively called “switching loss”.
To reduce the switching loss, a reduction in feedback capacitance of the MOSFET is effective. The reason is that the speed of switching becomes faster when the feedback capacitance is reduced, and thus the switching loss is reduced. The trench MOSFET essentially has a problem of large feedback capacitance and thus achieving a further improvement in switching frequency is difficult.
When the switching frequency of a VR is low (for example, about 300 kHz), a ratio of the conduction loss occupying loss of the VR is large. Thus, a trench MOSFET having a low ON resistance is advantageous. When the switching frequency is high (for example, 1 MHz or higher), the switching loss is dominant and a planar type MOSFET having a small feedback capacitance is advantageous.
As a structure capable of further reducing the feedback capacitance of the planar MOSFET, a structure (hollow-gate type gate planar MOSFET) in which a central portion of a gate electrode of a planar MOSFET is eliminated has been released (for example, see H. Esaki et al., “A 900 MHz 100 W VD-MOSFET with silicide gate self-aligned channel”, in Proc. IEEE IEDM' 04, 1984, pp. 447-450 (Non-Patent Document 4)).
The hollow-gate type MOSFET has a smaller overlap of a gate region and a drain region as compared with conventional planar MOSFETs, and thus the feedback resistance can be largely reduced.
Meanwhile, although the hollow-gate type planar MOSFET has a feature of small feedback capacitance, there is a problem of a large ON resistance. It has been known as means of improving the large feedback resistance that a second gate electrode is provided between gate electrodes of the hollow-gate type MOSFET and a positive voltage is applied to the second gate electrode, thereby reducing the ON resistance (for example, see Japanese Patent Application Laid-Open Publication No. 57-141964 (Patent Document 3) and Japanese Patent Application Laid-Open Publication No. 6-283718 (Patent Document 4)).